When physicians recommend a stress test, they are usually referring to an exercise electrocardiogram (EKG). This test measures the heart's electrical activity before, during and after exercise. However, technically speaking, a stress test is any test that is performed in conjunction with exercise. Thus, a nuclear stress test usually refers to stress testing that is performed in combination with a nuclear imaging test, such as SPECT scan or a PET scan.
In general, a nuclear stress test is more accurate and provides more information than a standard exercise EKG. However, these tests do have drawbacks. They are more expensive and require more time, and there is exposure to a small amount of radioactive substances.
Because of cost issues and availability of radioactive tracers, the SPECT stress test is the more common of the two. During a SPECT stress test, the patient is injected with a very small, (of negligible harm) amount of a radioactive (radionuclide) substance, such as thallium.
Once in the patient’s body and taken by the heart, this substance emits rays that can be detected by a special gamma camera. The rays allow the camera to produce clear pictures of heart tissue on a video monitor. These pictures show contrasts between light and dark spots, which can indicate areas of damage or reduced blood flow that are present before, during and after exertion.
During a PET stress test, the radioactive tracer (usually rubidium-82) is attached to a molecule such as glucose. When the glucose is absorbed into the heart tissue, special sensors detect the rate of absorption and the degree of absorption. This allows physicians to evaluate the metabolic health of the tissue.
A nuclear stress test is often performed in addition to the procedures that come as a part of a standard stress test. Aside from some possible discomfort as the radionuclide substance is injected (twice), this is a painless test. Patients are generally asked not to eat or drink anything for four to six hours before the test, and to wear comfortable clothes and shoes for exercising. The time needed for the test will vary because different facilities use slightly different strategies for taking pictures of the heart at rest and during some form of exercise. Patients are encouraged to speak with their physician about the length of the test and any preparatory instructions. After the test, they can immediately return to their usual daily activities.
Depending on the results of the nuclear stress test, the physician is often able to make a diagnosis and treatment plan for the patient. Further testing or procedures may need to be done.
About nuclear stress tests
During a traditional stress test, the heart’s electrical activity is measured by electrocardiograms (EKGs) both at rest and under exertion.
A nuclear stress test is a noninvasive test that combines exercise with a form of nuclear imaging such as a SPECT scan or PET scan. In either case, the test can reveal more detailed information than a conventional exercise EKG test.
During a SPECT exercise test, a patient is injected with an isotope, usually thallium but sometimes a technetium labeled agent such as sestamibi or tetrofosmin, to take pictures of the heart muscle. The isotope is absorbed by the heart muscle (myocardium), allowing physicians to use a special camera to obtain images of the heart. By studying the distribution of the isotope into the heart muscle, physicians can gauge the health of the heart and adequacy of coronary blood supply at rest and under stress.
A nuclear stress test can help measure:
Extent of artery blockage
Prognosis of patients who have recently suffered a heart attack (myocardial infarction)
Effectiveness of cardiac procedures previously performed, such as coronary stenting (in which a wire mesh metal tube is inserted into an artery to keep it open), or balloon angioplasty (in which a balloon-tipped catheter is used to push plaque back against artery walls, improving blood flow)
Cause(s) of chest pain
Level of exercise that a patient can safely perform
A nuclear stress test is not appropriate for everyone. Patients who may not be candidates for this test include those with the following conditions:
Inflammation of the heart muscle (myocarditis)
Recent pulmonary (lung) infection
Coarctation of the aorta
Pregnancy
Severe narrowing (stenosis) of the aortic valve
Severe heart failure
Cardiac PET scans, another type of radionuclide imaging test, can determine the presence and extent of coronary artery disease. It is an excellent tool for determining whether portions of the heart muscle are still viable (living and functioning). It is also very accurate, and it has shown it to be more sensitive than thallium stress testing. Studies have found that thallium stress testing may overestimate the extent of heart damage by as much as 30 percent of the time. PET scans may be recommended when other non-invasive tests (e.g., echocardiogram) do not yield a definite diagnosis.
About nuclear imaging
Through a technique known as single photon emission computed tomography (SPECT), a special gamma camera detects the rays emitted from a radionuclide substance (e.g., thallium) that is injected into the body. These rays produce clear images of heart tissue on a video monitor. A physician evaluates the heart’s function after exertion and at rest.
Because the radionuclide substance is absorbed by healthy tissue at a different rate than by diseased or damaged tissue, these images can give the physician specific information about the function and viability of the heart muscle and its ability to keep up with the increased demands of physical stress. “Cold spots,” or areas of disease or damage, will usually indicate lack of blood flow, vessel obstruction and/or muscle scarring. These tests are used to help physicians select candidates for surgery and/or catheter-based procedures, and also to measure the extent of a recent heart attack.
During hard exercise, heart muscle that cannot obtain adequate blood flow (perfusion) will show less radioactivity – producing less of a contrast on the monitor – than heart muscle that is receiving normal perfusion. Results can indicate any of the following:
If the test is normal during times of both exertion and rest, then blood flow is adequate and there is no scarring of the heart muscle.
If the test showed the same abnormality during both exertion and rest, then this part of the heart has most likely been scarred and damaged by a previous heart attack.
If the test is normal during rest, but abnormal during stress or exertion, then a blockage in one or more arteries may exist.
If there is a resting abnormality that becomes worse during stress, this indicates both heart attack and reversible ischemia
Patients who are unable to exercise may receive a pharmacological stress test. In this test, the patient is injected with a medication such as dobutamine. The medication causes the heart to react as if the person were exercising, though the patient is actually at rest. Other medications that perform this function include adenosine and persantine. Some drugs increase the heart rate, and other drugs increase blood flow by causing the coronary arteries to widen.
Before, during and after nuclear stress tests
Patients are advised to refrain from eating or drinking for about four to six hours before the test. Physicians find it easier to evaluate the heart’s performance when the patient’s stomach is empty, and patients are less likely to experience nausea or cramping during exertion. Physicians may also advise patients to refrain from taking caffeine and certain medications in the days or hours leading up to the test. These items can affect the results of the test. Patients are advised to wear comfortable, loose clothing and appropriate exercise shoes. Also, it may be necessary for the patient to sign a consent form before beginning the test.
Aside from the quick, minor discomfort that patients may feel when injected with the radionuclide substance, this test is considered to be relatively safe and painless. It can take place in a physician’s office, hospital or outpatient medical facility.
Before the nuclear component of the nuclear stress test is performed, preparations are made for the standard stress test. First, the patient is prepared for an electrocardiogram (EKG). The patient removes his or her clothes from the waist up, and a nurse or technician thoroughly cleans about a dozen small areas on the patient’s body – usually on the chest, arms and legs. Usually, the technician will apply a conductive gel to these areas. About 10 to 12 small metal devices (electrodes) are then taped to those clean areas of the skin and attached by wires to the electrocardiograph machine.
The next step varies, depending on the facility where the test is done and whether the patient is scheduled for an exercise stress test (which involves physical exertion) or a pharmacological stress test (which involves a medication that mimics the effects of exertion on the heart).
Regardless of how the heart rate is elevated, all patients will have pictures taken after stress has been placed on the heart through either exercise (usually on a treadmill) or medication. Once the desired heart rate has been reached, the thallium (or technetium or sestamibi) is injected. The patient will then be taken to another room to lie down on a table under a gamma camera that will take pictures of the heart. The first part of the test takes about 15 to 30 minutes.
After a few hours, when the heart rate is back to normal, additional pictures of the heart are taken. This measures the redistribution of thallium through the heart muscle.
Alternatively, the facility may first inject the patient with one substance, take resting pictures, increase the patient’s heart rate (through exercise or medication) and then inject the person with another substance before taking pictures again. In all, the test may take a total of two to two and a half hours.
At other facilities, patients may be asked to complete the exercise/medication component and take pictures on one day, then come back on another day to take pictures of the heart at rest. In some cases, a second injection of thallium will be administered, in conjunction with oral nitroglycerin, to improve the accuracy of the test. To plan their time accordingly, patients are encouraged to speak with their physician about how long the test will take.
The reason for the pause between testing and additional images is to improve the test. Studies have shown that the period of time between the exercise and the imaging will affect the test results. Some studies have shown that the best results are obtained when three image sets are taken: during stress, during the post-stress redistribution period, and after reinjection. By comparing the three sets of images, physicians can tell which parts of the heart muscle absorbed the isotope and how deeply it penetrated.
After the test, the patient is usually advised that he or she can eat, drink and resume normal activities immediately.
A physician may be able to provide the patient with a preliminary report at the end of the test, and a more complete report within a day. Based on the physician’s evaluation of how efficiently the heart is able to regulate blood flow at rest and during physical stress, a diagnosis can be made. The physician will then discuss his or her treatment plan for the patient, which may include additional procedures or tests (e.g., cardiac catheterization).
Benefits and risks
Nuclear stress tests can provide physicians with detailed information about at the heart’s structure and function. They can evaluate how well the heart is pumping blood as well as the presence of any damaged heart muscle. It also can detect blockages or narrowing of arteries due to coronary heart disease.
Nuclear imaging tests are considered safe with very few complications. In rare cases, patients may develop skin rashes from the radioactive tracer, large changes in blood pressure, difficulty breathing or arrhythmias. The amount of radiation exposure from the tracer does not pose a threat to patients.
In some cases, the test may yield a false positive result – indicating a heart defect when there is none present. This may be due to interference with the heart image from other non-heart tissues, such as breast tissue in women. If physicians suspect non-heart tissue is affecting the accuracy of the test, additional testing such as stress echocardiogram or cardiac catheterization may be recommended.
Questions for your doctor
Preparing questions in advance can help patients to have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions related to the nuclear stress test:
Why do I need a nuclear stress test?
What do expect to see in the results?
Will I feel any pain or discomfort during the test?
Is it safe for me to take a test that requires exercise if I have an existing heart condition?
Do I need to alter my diet or stop taking any medications prior to the test?
How long will the test take?
Can it be performed all in one day?
When and from whom will I learn the results?
Will I exercise or receive medication to simulate exercise?
